This invention relates to a fuel control apparatus for an engine in a vehicle for burning fuel at an optimum air-fuel ratio.
FIG. 5 shows a prior-art fuel control apparatus for an engine. In FIG. 5, numeral 1 designates an engine, numeral 2 an intake manifold, numeral 3 a fuel injection valve mounted in the intake manifold 2 to surround the vicinity of the intake port of the engine 1, numeral 4 a surge tank of intake air pressure provided between the intake manifold 2 and an intake conduit 5, numeral 6 a throttle valve provided in the intake conduit 5, and numeral 7 an air flow sensor provided near the upstream end of the intake conduit 5 and provided, for example to be disposed in a ring-shaped air duct. The air flow sensor 7 is an air flow rate measuring instrument for measuring, on the basis of a heat dissipating principle, the weight, the temperature and the density of the intake air and provides the same as output data. Numeral 8 indicates a controller which calculates and decides the optimum fuel injection amount in accordance with the output of a rotary sensor 9 for detecting the rotating speed of the engine 1 and the output of the air flow sensor 7. The controller 8 generates a signal having a pulse width corresponding to the optimum fuel injection amount so as to operate the fuel injection valve 3 in accordance therewith.
The controller 8 comprises, as shown in FIG. 6, of a computer. More specifically, numeral 81 designates an analog/digital converter (hereinafter referred to as "an A/D converter") for converting the analog output of the air flow sensor 7 into a digital signal for calculation processing, numeral 82 an interface circuit for inputting the digital output of the rotary sensor 9, numeral 83 a microprocessor (hereinafter referred to as "a CPU") for calculating an optimum fuel supply amount in accordance with the outputs of the A/D converter 81 and the interface circuit 82, numeral 84 a memory (hereinafter referred to as "a RAM") for temporarily storing various data (including the abovementioned outputs) used at the calculating time, numeral 85 a memory (hereinafter referred to as "a ROM") for storing data such as calculating sequence, and numeral 86 an amplifier for amplifying a fuel supply amount signal output from the microprocessor 83. Next, the operation will be described.
When the engine 1 is operated in any operating state except the vicinity of full open (WOT) of the throttle valve 6, the output from the air flow sensor 7 becomes a waveform which includes a normal ripple as shown by a curve (a) in FIG. 7. When the area covered by the waveform is calculated, the true intake air weight can be obtained. Thus, when the microprocessor 83 controls the drive pulse width of the fuel injection valve 3 in accordance with the value produced by dividing the intake air amount by the rotating speed of the engine, it can provide a desired air-fuel ratio.
However, in an engine having less than four cylinders, the output waveform of the air flow sensor 7 becomes as shown by a curve (b) in FIG. 7 due to the reverse-flow from the engine 1 in the special rotating speed range (generally in a range of 1000 to 3000 r.p.m.) near the WOT, and the area indicated by the hatched portion is excessively added to the true intake air weight.
This is due to the fact that the hot-wire type air flow sensor 7 detects and outputs as the intake air amount a value irrespective of the air flowing direction.
The detecting error of the sensor 7 caused by the reverse-flow depends, as shown in FIG. 8, upon the rotating speed of the engine, and normally occurs from when the vacuum in the intake conduit is near -50 mmHg and arrives at 50% of the maximum in the WOT range.
When the fuel supply amount is calculated and injected with respect to a value which contains such a large error, the air-fuel ratio becomes very rich, the combustion in the engine becomes unstable, thereby becoming practically impossible to use. Heretofore, as shown in FIG. 9, the upper limit value (designated by a broken line) is set in the maximum air amount determined for the engine in the area a that the error occurs by reason of the reverse-flow, and stored in the ROM 85, and the detected value of the air flow sensor 7 exceeding this limit value is clipped by the upper limit value as shown by (b) in FIG. 7, thereby suppressing the excessively dense air-fuel ratio.
Since the prior-art fuel control apparatus for the engine is composed as described above, the upper limit value of the intake air amount must be set to match the intake air amount characteristic of the engine to be countermeasured at ambient temperature, and the upper limit value must become the upper limit of the mass flow rate at the ambient temperature.
However, if the engine is operated, for example, with a high load in the state that the intake air temperature is high, the output level of the air flow sensor 7 does not reach the average value at the predetermined upper limit value as shown by (c) in FIG. 7 due to the reduction in the air density. Thus, the average value of the output level which contains the reverse-flow is used in the calculation of fuel as it is, with the result that the air-fuel ratio is shifted to the rich side. On the other hand, when the temperature of the intake air is low, the air density increases. Thus, the actual intake air amount of the engine is increased to become larger than the upper limit value as shown by (d) in FIG. 7, and the air fuel ratio is shifted to the lean side. Therefore, the air-fuel ratio varies with respect to the intake air temperature as shown in FIG. 10. In other words, when the upper limit value of the intake air amount is determined by the engine near the ambient temperature, there arises a problem that the error of the air-fuel ratio increases with the increase in atmospheric temperatures.